1. Field of the Invention
This invention relates to vanadium and sulfur compound corrosion-resistant ceramic coatings. More particularly, this invention relates to scandia-stabilized zirconia coatings and composites formed with them.
2. Description of the Prior Art
As demands increase on fuel resources and as manufacturing techniques become more complex, industrial equipment is exposed to fuels and materials which contain corrosive contaminants. These corrosive contaminants can cause extensive damage to the surface and structure of industrial equipment, particularly to all types of motors, turbines, engines, furnaces, stacks, fluidized beds and the like.
In addition, it is often necessary to protect industrial parts from damage and fatigue caused by operation at high temperatures. Ceramic coatings have been used to protect exposed surfaces from heat deterioration and corrosion.
Vanadium and sulfur compounds are particularly virulent corrosive materials which are found in many fuels and raw materials. Sulfur and vanadium compounds react during combustion to produce high temperature vanadium and sulfur oxide gases within the machine or engine, and also react with sodium or sodium oxide (Na being a contaminant found in virtually all environments and ingested into the engine or machine) to deposit thin films of molten sodium vanadates and sulfates on the hot machine parts such as turbine blade or piston surfaces. It is believed that vanadium is oxidized to V.sub.2 O.sub.5 in gas turbine combustion and that the sulfur compounds are present as oxides, acids and as free sulfur.
Because vanadium pentoxide (V.sub.2 O.sub.5) is an acidic oxide, it reacts with Na.sub.2 O (a highly basic oxide) to form a series of compounds in which the acidic nature of the compounds decreases with the V.sub.2 O.sub.5 /Na.sub.2 O ratio from Na.sub.2 V.sub.12 O.sub.31 (most acidic) to Na.sub.3 VO.sub.4 (least acidic). Each of the acidic oxides can cause damage to machine parts. The oxides are formed as follows: EQU Na.sub.2 O+6V.sub.2 O.sub.5 =Na.sub.2 V.sub.12 O.sub.31 (vanadium bronze I) EQU Na.sub.2 O+3V.sub.2 O.sub.5 =2NaV.sub.3 O.sub.8 (vanadium bronze II) EQU Na.sub.2 O+V.sub.2 O.sub.5 =2NaVO.sub.3 (sodium metavanadate) EQU 2Na.sub.2 O+V.sub.2 O.sub.5 =Na.sub.4 V.sub.2 O.sub.7 (sodium pyrovanadate) EQU 3Na.sub.2 O+V.sub.2 O.sub.5 =2Na.sub.3 VO.sub.4 (sodium orthovanadate)
The combination of both vanadium and sulfur compounds is particularly destructive to machine parts. Rahmel, in the "Proceedings of International Conference on Ash Deposits and Corrosion Due to Impurities in Combustion Gases," Byers, Editor, p.185, Hemisphere, Washington, D.C. (1978), reports electrochemical studies of the corrosion of superalloys in molten sulfates containing different concentrations of the different vanadium compounds which indicate that the corrosiveness of the vanadium compounds decreases from V.sub.2 O.sub.5 to NaVO.sub.3 to Na.sub.3 VO.sub.4.
With the increasing threat of an energy crisis, pulverized coal is becoming more attractive as a fuel source, but coal also contains vanadium and sulfur compounds which can destroy the insides of furnaces, including incinerators, or other combustion equipment, stacks servicing that equipment or furnaces, and the containers of fluidized beds.
Ceramic coatings have been used to try and protect surfaces from corrosion and to provide, when appropriate, a thermal barrier. Zirconia is a ceramic with excellent heat insulating properties as well as excellent resistance to corrosion by vanadium and sulfur compounds. Zirconia appears well suited to act as a protective coating to materials exposed to an atmosphere containing corrosive vanadium and sulfur compounds, but pure zirconia undergoes a catastrophic tetragonal-to-monoclinic phase structure change at 1000.degree.-1100.degree. C. This change results in an approximately 4% change in volume of zirconia. Such a volume change in the working parts of a machine, such as an engine, as it cycles through that temperature range is likely to result in flaking or deterioration of coatings formed from zirconia. These flakes would be calamitous to the machine.
It is known that zirconia can be stabilized to the tetragonal crystal structure by the addition of stabilizer compounds such as 5-20 wt-% of calcia (CaO), magnesia (MgO), or yttria (Y.sub.2 O.sub.3). Andreev et al., J Crystal Growth, V. 52, pp. 772-776 (1981), reports using zirconia crucibles stabilized with several different oxides, including scandia, to grow semiconductor crystals. Stabilized zirconia has also been used to form reaction vessels, electrodes, and electrolytes for electrochemical reactions.
NASA and others are developing zirconia "thermal barrier" coatings for use on gas turbine blades and diesel engine pistons. These coatings are expected to substantially increase engine thermal efficiency. Siemers et al., in U.S. Pat. No. 4,328,285, describes some of the prior art attempts to coat engine parts with ceramic base materials, and Siemers teaches using cerium oxide or ceria stabilized zirconia ceramic coatings to protect turbine and engine surfaces exposed to vanadium and sulfur compound corrosion.
The zirconia thermal barrier coatings have not been completely successful because stabilized zirconia has been found to react with, and be quickly degraded by traces of sulfur, and sulfur and vanadium compounds present in many commercial and industrial grade petroleum fuels. One of the inventors, together with colleagues, has reported in R. L. Jones, C. E. Williams, and S. R. Jones, J. Electrochem. Soc. 133, 227 (1986); R. L. Jones, S. R. Jones, and C. E. Williams, J. Electrochem. Soc. 132, 1498 (1985); and R. L. Jones and C. E. Williams, Surface and Coatings Tech. 32, Nr. 1-4, 349 (1987), that the instability of the zirconia is traceable to the leaching of the stabilizer from the coating and not to the zirconia itself. A stabilizer which will not be leached from the coating on high temperature exposure to vanadium and sulfur compounds is needed before zirconia can be successfully used as a corrosion resistant coating.